As is well known, a fuel cell is a system for generating electric energy through an electrochemical reaction between oxygen and hydrogen contained in hydrocarbon materials such as methanol, ethanol, and natural gas.
Recently developed polymer electrolyte membrane fuel cells (hereinafter, referred to as PEMFCs) have excellent output characteristics, low operating temperatures, and fast starting and response characteristics. Therefore, PEMFCs have wide ranges of applications including use as mobile power sources for vehicles, as distributed power sources for homes or buildings, and as small-sized power sources for electronic apparatuses.
A fuel cell system employing the PEMFC scheme basically requires a stack, a reformer, a fuel tank, and a fuel pump. The stack constitutes an electricity generator set having a plurality of unit cells. The fuel pump supplies fuel from the fuel tank to the reformer. Then, the reformer reforms the fuel to generate hydrogen gas that is supplied to the stack.
Since the reformer generates the hydrogen gas from the fuel through a catalytic chemical reaction that requires thermal energy, the reformer should include a heat source section for generating the thermal energy and a reforming reaction section for absorbing the thermal energy and generating the hydrogen gas from the fuel.
In the reformer of a conventional fuel cell system, since the heat source section and the reforming reaction section are formed in a separate vessels and the heat is distributed through a pipe, the heat exchange between the parts is not directly performed and thus it is disadvantageous in heat delivery. Since the respective parts are distributed, it is also difficult to make a compact fuel cell system.
In a conventional fuel cell system, since the fuel supplied to the reformer can be preheated using an additional preheating device, much energy is spent in preheating the fuel, thereby deteriorating performance and thermal efficiency of the entire fuel cell system.
In addition, a conventional fuel cell system generally has a plurality of carbon monoxide reducing sections for reducing the concentration of carbon monoxide through a water-gas shift (WGS) reaction of the hydrogen gas generated from the reforming reaction section or through a preferential CO oxidation (PROX) reaction of carbon monoxide contained in the hydrogen gas.
In a conventional reformer, since the reforming reaction section and the carbon monoxide reducing sections are provided separately and the hydrogen gas generated from the reforming reaction section is supplied to the carbon monoxide reducing sections, it is difficult to make a compact system. In a conventional reformer, the preferential CO oxidation of the carbon monoxide reducing sections is an endothermic reaction. Accordingly, when the heat generated through the endothermic reaction and the heat generated from the heat source section are more than 200° C., hydrogen is combusted with the heat, thereby deteriorating the efficiency of the reformer.